1. Field of the Invention
This invention relates to circuits for flash equipment and, more particularly, to circuits for flash equipment wherein the flash tube need only be flashed once at a time of simultaneous occurrence of the completion of charging a main flash capacitor and the closure of an X sync contact.
2. Description of the Prior Art
In order to ensure that the flash tube is fired only once at a given instant in time in order to derive a correct flash exposure after the main flash capacitor is charged to the normal firing voltage, it is known to employ a condenser as part of an initiating circuit. Thus, the charge on the condenser is discharged to actuate a trigger circuit in response to an AND signal occurring when the main flash capacitor is charged to the normal firing voltage and when the X sync contact is closed. An example of such a flash circuit is disclosed in U.S. Pat. No. 3,912,968.
Since the actuation of the trigger circuit is controlled by the AND signal, when the exposure time is prolonged to allow sufficient recharging of the flash capacitor and the initiating circuit condenser, the flash tube will be refired at a proper time at which the X sync contact is still closed.
To solve this problem, it has been proposed to connect a differentiation circuit between the aforesaid condenser and the trigger circuit as, for example, in U.S. Pat. No. 4,095,141. With this proposal, it is impossible to avoid the above-mentioned drawback completely because of its utilization of a discharging time of a condenser constituting part of the differentiation circuit, after which time the trigger circuit is rendered effective again. Further, there arises an alternate problem that, when the voltage of the main flash capacitor attains a predetermined level at a point in time during discharging of the differentiation circuit condenser, the latter starts to be charged again. Thus, there is a high possibility of unintentionally actuating the trigger circuit which leads to refiring of the flash tube.